1. (Field of the Invention)
The present invention relates to a light-sensitive composition suitable for preparing a presensitized plate for use in making a lithographic printing plate (hereunder referred to as "PS plate") and more particularly to a light-sensitive composition containing a photocrosslinkable polymer soluble in an aqueous alkaline solution and capable of getting swollen with the alkaline solution, which comprises cinnamate ester type photodimerizable groups and alkaline-soluble carboxyl groups in the main chain thereof and which has improved adhesion to an aluminum substrate.
2. (Description of the Prior Art)
Photocross-linkable materials (or light-sensitive resins) which are cross-linked by a ring-addition reaction have been well known in the art and these materials have been used as materials for forming images such as PS plates and Large Scale Integrated Circuit (LSI) elements. Among these photocross-linkable materials, useful ones are those having cinnamyl, cinnamylidene or chalcone groups carrying photocross-linkable unsaturated double bonds adjacent to an aromatic nucleus on the side chains or the main chain thereof and some of these have already been put into practical use. In particular, it is known that a light-sensitive polyester resin having, in the main chain of the molecule, cinnamic acid skeletal structures exhibits a rather high sensitivity. Such a light-sensitive polyester can be prepared by condensing a phenylene diacrylic acid or an alkyl ester thereof and glycol. In general, such a light-sensitive resin should be aqueous alkaline soluble in order to prevent environmental pollution from waste liqour when it is used as a PS plate by applying it on, for instance, an aluminum substrate. Moreover, it should be highly adherent to such a substrate as an aluminum plate to prevent images formed thereon from being peeled off by a brush or the like during development and from causing defects thereon and to impart sufficient printing durability to the final products such as lithographic printing plates. However, the aforementioned light-sensitive resins do not satisfy the foregoing requirements and, therefore, these resins have a limited application and cannot generally be used in obtaining light-sensitive layers of PS plates.
In order to eliminate such disadvantages, many attempts have been directed to the development of the improved light-sensitive resins. For instance, U.S. Pat. No. 4,640,887 (Japanese Patent Un-examined Publication (hereinafter referred to as "J.P. KOKAI") No. 60-191244) discloses the use of an alkaline soluble light-sensitive composition composed of polymeric light-sensitive compounds obtained by reacting a polyester prepolymer which has photodimerizable unsaturated double bonds adjacent to an aromatic nucleus in the main chain thereof, carboxyl groups on the side chains thereof and hydroxyl groups at the termini, with a chain extender having at least two functional groups capable of reacting with the hydroxyl groups of the prepolymer, such as diisocyanate compounds, diphenyl terephthalate, diphenyl carbonate and terephthaloyl-bis(N-caprolactam); and a polymeric light-sensitive compound obtained by reacting a polyester or polyurethane prepolymer which has photocrosslinkable unsaturated double bonds adjacent to an aromatic nucleus in its main chain and hydroxyl groups at the termini thereof with a chain extender such as dianhydride of pyromellitic acid or cyclopentane tetracarboxylic acid to introduce carboxyl groups into the side chains thereof.
However, the following complicated steps must be required to prepare the aforementioned light-sensitive polymeric compounds:
(1) reacting a polybasic carboxylic acid component such as a dicarboxylic acid having a light-sensitive unsaturated double bonds adjacent to an aromatic nucleus with a polyol component to form an oligoester having hydroxyl groups at the termini thereof;
(2) reacting the oligoester having hydroxyl groups at the termini with a cyclic acid anhydride while adjusting the ratio therebetween so that the hydroxyl groups at the termini of the oligoester certainly remains unchanged; then
(3) reacting the oligoester obtained in the step (2) with a chain extender capable of reacting with the hydroxyl groups of the oligoester to form a polymer. These steps require very delicate operations in selecting the concentrations of the chemicals to be reacted and reaction temperature and involve side reactions which provide by-products in addition to the desired product, which in turn causes gelation to form insoluble substances during development of the light-sensitive layer formed therefrom. Therefore, such a process cannot appropriately provide light-sensitive polymers having desired properties.